For the first time, a team of scientists led by Roger Kornberg has synthesized thiol-covered gold nanoparticles and, using ALS Beamlines 5.0.2 and 8.2.2 and SSRL Beamlines 11-1 and 11-3, conclusively ascertained their atomic structure (at 1.1 Å resolution). The gold–thiol nanoparticle consists of 102 gold atoms surrounded by 44 molecules of a thiol compound (para-mercaptobenzoic acid, or p-MBA). The central gold atoms are grouped in a fivefold symmetric packing arrangement known as a Marks decahedron, which is surrounded by additional layers of gold atoms in unanticipated geometries. The protective p-MBAs interact not only with the gold but with one another, forming a rigid surface layer. This research is a success on several levels.The group developed a technique that solves a previously unsolvable nanostructure. They delivered a very detailed atomic map of this structure, which itself reveals an unusual discovery: the discrete nature of the nanoparticle, which can be explained by the closing of a 58-electron shell.

Gold Gives Up Its Secrets

Gold has been a symbol of wealth, purity, and royalty for centuries. Alchemy, which reached its height during the Middle Ages, aimed to turn base metals such as lead into gold. It didn't succeed, but it did lay the foundation for modern-day chemistry.

Today, iridescent gold particles are used in biosensors, drug delivery, and many other applications, but for all gold's uses, the structure of tiny gold particles (nanoparticles) remains unknown. Is it amorphous or composed of discrete atomic arrangements of uniform size and structure? Roger Kornberg and his colleagues have laid that mystery to rest. They surrounded gold particles with a sulfur compound (a thiol) to make synthesis easier, and crystallized the particles. The structure was then determined by x-ray diffraction. A gold–thiol megamolecule was revealed, consisting of 102 gold atoms surrounded by 44 thiol molecules. We now know that a unique gold nanoparticle can be formed and maintained; the core is metallic and the surface a gold–sulfur polymer; and the particle's unique nature is determined not by a so-called magic number of gold atoms resulting from the closing of a geometric shell, but by the filling of an electronic shell. The 44 thiol molecules snag a gold electron and bind it, leaving the 58 remaining gold atoms to contribute one valence electron and create a filled shell.

Gold is a useful metal for nanoapplications. It is nonreactive, soft, and does not oxidize. Metallic gold nanoparticles are used as tracers, biosensors, and in drug delivery. However, traditional crystallography methods break down for particles at the nanoscale level, and methods of analyzing x-ray scattering data to determine structure from disorganized masses of nanoparticles have been unsuccessful. So gold's structure remained a mystery. Kornberg et al. solved this "nanostructure problem" by working it in reverse: they developed a procedure for synthesizing stable clusters of a uniform size, then used x-ray diffraction to reveal the precise location of the atoms.

View down the cluster axis of the two enantiomeric ("mirror-imaged") particles, denoting chirality. Gold atoms depicted as yellow spheres, sulfur atoms of p-MBA in cyan.

Coating gold with organic monolayers makes it easier to synthesize the nanoparticles. Gold nanoparticles coated with surface thiol layers have been studied in detail, making them a good choice for synthesis. The thiol-coated gold particles were crystallized from a solution containing 40% methanol, 300 mM sodium chloride, and 100 mM sodium acetate at pH 2.5. After synthesis, 15 separate crystals were examined.

The electron density map revealed 102 gold atoms and 44 thiol molecules. The clusters were entirely homogenous, and the numbers of gold atoms and thiols were precise. The Au102(p-MBA)44 structure revealed a Marks decahedron core of 49 atoms (with 4 atoms on the central axis). But there were also a couple of surprises: two 20-atom caps with fivefold rotational (C5) symmetry on opposite poles (expanding to 89 the number of gold atoms with C5 rotational symmetries), and a 13-atom equatorial band with no apparent symmetry. All of this is protected by a Au23(p-MBA)44 layer of gold–thiol oligomers. The number and geometry of the atoms in the equatorial band give chirality ("handedness") to the core. Gold–thiol bonding and interactions between the p-MBA molecules impart stability to the surface.

Packing of gold atoms in the nanoparticle. The Marks decahedron is in yellow, two 20-atom "caps" at the poles in green, and the 13-atom equatorial band in blue.

The existence of a discrete Au102(p-MBA)44 nanoparticle is an important discovery. ("Discrete" particles exist only for a certain number of atoms, not any number chosen randomly.) Discrete sizes have been traditionally explained by either geometrical or electronic shell closing. In this case, however, the arrangement of gold atoms, with polar caps and an equatorial band, argues against geometrical shell closing. Electronic shell closing provides a better explanation for the uniform size of these clusters. The results seem to bear out the theory that each of the 44 sulfur atoms attracts a gold electron (5d106s1) into a localized orbital, leaving 58 gold atoms, each contributing one valence electron, to form a filled shell.

The gold–thiol experiment is not only a remarkable success but was consistently repeatable, with each of the 15 crystal samples having 102 gold atoms and 44 thiols and the unique 13-atom equatorial band. This research significantly adds to the understanding of nanoparticles and gold–thiol interactions and to the development of practical applications for monolayer-coated gold nanoparticles, and has delivered a reliable nanoscale structure determination technique.

Research Funding: U.S. Department of Energy, Offices of Biological and Environmental Research and Basic Energy Sciences (BES); and National Institutes of Health, National Institute of General Medical Sciences and National Cancer Institute. Operation of the ALS and SSRL is supported by BES.